The present invention relates to liquid rheostats, and more particularly to an improved liquid rheostat having cylindrical electrodes which are surrounded within a solution of electrolyte by an electroconductive element. The element provides portions of current paths for diffusing the current between the electrodes to thereby substantially reduce current concentration and electrode erosion.
Liquid rheostats are known to vary the speed of induction motors by varying the resistance in the rotor circuit. Such rheostats typically take the form of a container having electrodes disposed within an electrolyte solution held by the container. When the solution is varied in depth, the resistance between the electrodes is varied, i.e., as the solution level decreases the resistance increases and as the solution level increases the resistance decreases. Additionally, prior art liquid rheostats have generally operated under the principle that electrode spacing is responsible for a change in resistance. Such rheostats have a number of significant drawbacks.
First of all, prior art liquid rheostats are generally quite large and bulky for the reason that large electrode surface areas are required to prevent current concentrations on the electrodes from becoming excessive. High current concentrations will accelerate erosion of the electrodes. Thus, prior art devices have provided electrodes having large surface areas in order to minimize current densities. However, such large liquid rheostats are bulky and heavy in addition to being expensive to construct. Furthermore, large rheostats are undesirable for applications such as underground pumping stations where space is often limited.
Because prior art liquid rheostats operate on the principle that electrode spacing, and solution level are responsible for resistance between electrodes, various complex electrode configuration have been proposed. For instance, it has been suggested to provide tapering electrode edges in order to vary the distance between electrodes as the electrolyte solution is raised or lowered in order to vary the resistance. Such complex shapes may be difficult and expensive to manufacture. Additionally, current tends to concentrate along sharp or angular electrode edges, thus accelerating erosion and shortening electrode life.
Another problem present in prior art liquid rheostats resides in the methods used to cool the electrolyte solution. Because current passes through the solution, heat is generated which must be removed. For instance, in pumping installations where resistance in the rotor circuit of a pump motor is controlled by means of a liquid rheostat, liquid electrolyte is cooled by connecting the rheostat through a series of supply and return pipes to a heat exchanger which encircles the discharge pipe of the pump. Because the liquid rheostat is separately situated from the discharge pipe, such pumping installations may require a significant amount of space. If the pumping installation is to be underground, as is often the case, the requirement of a large amount of space is undesirable from a construction and cost standpoint.